So far as I know, correct me if I'm wrong, it is the following lines at the beginning of each file which sets the HTML as XHTML 1.1
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html
PUBLIC "-//W3C//DTD XHTML 1.1//EN"
"http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">
I haven't bothered to find how to make the HTML editor I use produce the files as .xhtml. But they still work and validate to W3C standards and with EPUB-Checker.
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